1. Field of the Invention
This invention relates to flow-velocity measuring apparatus of the type wherein a cylindrical object in a stream of flowing fluid produces a wake including a distinct pattern of vortices known as the Karman vortex street, and wherein the rate of production of vortices is detected ultrasonically. The vortices are shed alternately from opposite sides of the object in a periodic manner. There is a definite relationship between the frequency f of shedding of the vortices, the velocity v of the stream, and the height h of the object in the direction transverse to the direction of the stream of flowing fluid, expressed by: EQU f = Kv/h
Where K is a constant for flow within a range of velocities. Within this range, it is possible to determine the flow velocity v by measuring the frequency f of the generation of vortices.
More particularly, this invention relates to ultrasonically detecting the generation of vortices by making use of the vortex property of oppositely directed velocity components to frequency-or pulse-modulate an ultrasonic signal propagated across the flowing fluid in which the vortices are generated.
2. Description of the Prior Art
Various arrangements have been proposed for ultrasonically detecting the rate of production of Karman's vortices. In one such arrangement, disclosed in U.S. Pat. No. 3,680,375 to Robert D. Joy and in U.S. Pat. Nos. 3,693,438 and 3,756,078 to Hiroo Yamasaki, et al, an ultransonic generator and receiver are positioned downstream from a vortex generating element to propagate an ultrasonic signal transversely through the vortex street trailing from the element. The ultrasonic signal is frequently modulated by the passage of Karman's vortices. This signal is demodulated to yield a signal varying with the rate of production of Karman's vortices.
In another prior art arrangement, disclosed in Japanese Patent Office Prepublication no. 48-30961, an ultrasonic signal is propagated along a stream in front of a vortex generation element and is modulated by fluid flow developed in front of the vortex generation element in correspondence with the production of vortices downstream therefrom.
Such prior art apparatus of the general type referred to above has not been fully satisfactory under certain conditions of flow. For example, at low flow velocities, noise components of flow begin to approach in significance the signal components of alternating flow caused by vortex production, and accurate measurement becomes difficult because the ultrasonic signal is modulated by the turbulence and noise in the flowing fluid. Accordingly, insufficient flow measurement sensitivity is available over a wide range of flow velocities.